Thanks everyone for your help.
I went to Staffordshire classic bike show on Saturday and eventually found one carburetor. Just one....
No identification markings but a mikuni nevertheless.
I had a closer look at the carburetor yesterday and turns out its brand new! No petrol varnishing or muck in it whatsoever. I set the float height and fitted it.
Absolutely brilliant result, the bike runs as sweet as a nut. Chuffed to bits that the problem is now sorted.
Cost of carburetor from the autojumble £35. Bargain